I expect we've all seen it ... a lovely terrain piece that is slightly spoiled by the base having warped after the texture, paint, grass, etc has been applied. I have a couple of such items in my collection. So I set about experimenting with ways to prevent it. This is what I've learned ...
The base for the field (from my previous post) ready to be pre-curved. Ideally I would not apply any texture first in case it fell off when the board is being curved, but in my enthusiasm to get it done, I forgot. Doh!? Approx size 26cm x 21cm, of 3mm MDF.
it carefully, upside-down, between two books. The edges just need to reach the
books. I've found that for rectangular boards you should pre-curve along the longest dimension.
a small, solid item in the middle of the board – it just helps to focus the
force of the weight that goes on top.
stack PLENTY of heavy books on top. MDF will take quite a bit of weight! This must have been in excess of 5Kg's.
board is 26cm wide and is bent by approx, 1cm.
for at least 24 hours, preferrably 48 hours. Then remove the books and do whatever you’re planning to do with the
board right away, e.g. sand, paint, etc. After removing the books and turning the board back the right way up it should still have a slightly raised arch. The 'pre-curving' should offset the
warping effect of paint/glue. Occasionally the board stays slightly ‘arched’
but they seem to flatten after a while.
Make sure the finished items are stored flat otherwise they can be prone to warping even after some time.
It's not an exact science but it does seem to work quite well compared to other techniques such as scoring or painting the opposite side.
Hope that's useful.